Obviously in the long term use a/freeze 50/50, but water will not just boil and evaporate in a sealed system, it is most likely a dodgy gauge, or sender.
Maybe to ease your mind check the actual temp with a infra red gun, say on the stat housing, just a few degrees rise can have a massive effect on the gauge reading, but water won't boil till well over 100C anyway, it would be a good idea just to run some Wynns rad flush stuff in the system for a few hundred miles,(not the 10 minute idle it says on bottle), this is what I do, it leavesd a congealed mess in the bottom of the expansion tank, which is all the crap it takes out, sometimes as much as 1/2 MUG full!!
It can then be washed out when you take the tank off, this will also free anything in the heater, as winter approaches!!
tabetha